The peak-to-average power ratio (PAPR) of a signal is defined as the ratio of the signal's peak power to its average power. PAPR is the square of the crest factor, which is defined as the signal's peak amplitude divided by its average value (i.e., its root-mean-squared, or RMS, value). A high PAPR and corresponding high crest factor reduce power-amplifier (PA) efficiency and increase the dynamic range for transmit-signal-processing stages, thus complicating the design of those stages. Dynamic range refers to the ratio of the maximum signal magnitude to the minimum signal magnitude. High PAPR is a known challenge in orthogonal frequency-division multiplexing (OFDM).
Accordingly, it is desirable to reduce the PAPR of digital baseband transmit signals, such as digital baseband OFDM transmit signals. The PAPR may be reduced by reducing the dynamic range.
One approach to dynamic range reduction, and thus to PAPR reduction, is clipping and filtering. FIG. 1 is a block diagram of PAPR reduction circuitry 100 that clips and filters a digital baseband signal provided as input. The digital baseband signal is a quadrature-amplitude-modulation (QAM) baseband signal with in-phase (Iin) and quadrature (Qin) components. This input signal is provided to an absolute-value module 102 and a multiplier 106. The absolute-value module 102 determines the magnitude |in| of the input signal and provides the magnitude |in| to a lookup table (LUT) 104. A clipping level A is also provided to the LUT 104. The LUT 104 uses the magnitude |in| and the clipping level A to perform a lookup that returns a clipping factor c. The clipping factor c may be determined using the formula:
                    c        =                  {                                                                                          1                    ,                                                                                                                                                            i                        ⁢                                                                                                  ⁢                        n                                                                                    ≤                    A                                                                                                                                          A                      /                                                                                                i                          ⁢                                                                                                          ⁢                          n                                                                                                              ,                                                                                                                                                            i                        ⁢                                                                                                  ⁢                        n                                                                                    >                    A                                                                        .                                              (        1        )            
The multiplier 106 multiplies the input signal by the clipping factor c, thereby clipping the input signal. A low-pass filter (LPF) 108 filters the clipped input signal, resulting in an output signal of the PAPR reduction circuitry 100. The output signal is a QAM baseband signal with in-phase (Iout) and quadrature (Qout) components.
The clipping-and-filtering technique performed by the PAPR reduction circuitry 100 involves a large number of multiplications, because the QAM baseband signal is complex-valued. Accordingly, there is a need for clipping-and-filtering techniques that are computationally simple.